This invention relates to an improved process of papermaking wherein polymer combinations of cationic and anionic starches having a select zeta potential are added to the stock or furnish in the wet end to provide improved retention as well as drainage and strength properties.
The term xe2x80x9cpaperxe2x80x9d as used herein, includes sheet-like masses and molded products made from fibrous cellulosic material, which may be derived from natural sources, synthetics such as polyamides, polyesters, rayon and polyacrylic resins as well as from mineral fibers such as asbestos and glass. In addition, paper made from combinations of cellulosic and synthetic materials are applicable herein. Paperboard is also included within the broad term xe2x80x9cpaperxe2x80x9d.
Papermaking, as it is conventionally known, is a process of introducing an aqueous slurry of pulp or wood cellulosic fibers (which have been beaten or refined to achieve a level of fiber hydration and to which a variety of functional additives can be added) onto a screen or similar device in such a manner that the water is removed, thereby forming a sheet of the consolidated fibers, which upon pressing and drying can be processed into dry roll or sheet form. Two well known papermaking processes involve the Fourdrinier machine, the most common, and the cylinder machine. In the Fourdrinier and multicylinder operations, and in other machine operations, as typical in papermaking, the feed or inlet to the machine is an aqueous slurry or water suspension of pulp fibers which is provided from what is called the xe2x80x9cwet endxe2x80x9d system. In the wet end, the pulp along with other additives are mixed in an aqueous slurry and subjected to mechanical and other operations such as beating and refining to improve interfiber bonding and other physical properties of the finished sheet. Additives commonly introduced along with the pulp fibers are pigments such as titanium dioxide, mineral fillers such as clay and calcium carbonate and other materials introduced into paper to achieve such properties as improved brightness, opacity, smoothness, ink receptivity, fire retardance, water resistance, increased bulk, etc.
It has been known to add various materials, including starch, to the pulp, or stock in the papermaking process or prior to the formation of the sheet, to aid in retention, drainage and strength properties.
Starch has been used in the paper industry for many years and, in fact, is the second largest volume raw material component in paper. Starches perform a number of functions in papermaking including strength improvement, increased drainage and increased retention of fibers, fines and other components on the wire. Both unmodified and modified types have been used.
Anionic and cationic starches as well as amphoteric starches have long been used as additives in papermaking for their contributions to strength and pigment retention in the paper. See, for example, U.S. Pat. No. 3,459,632 issued to C. Caldwell et al. on Aug. 5, 1969 and U.S. Pat. No. 3,562,103 issued to K. Moser et al. on Feb. 9, 1971. More recent patents involving the use of starches in papermaking include U.S. Pat. No. 4,876,336 issued to D. Solarek et al. on Oct. 24, 1989 which discloses the use of amphoteric starch derivatives and U.S. Pat. No. 5,129,989 issued to S. Gosset et al. on Jul. 14, 1992 which discloses the use of cationic and anionic starches in separate additions.
Despite the various and well known use of different starches in papermaking, there is the continuing need and desire to provide improved papermaking properties and especially improved retention.
Now, it has been found that significantly improved retention properties can be obtained in the process of papermaking by the addition or combination of cationic starch and starch phosphate having a select zeta potential, to the stock or furnish in the wet end.
More particularly, this invention involves the process of making paper comprising adding to the paper stock or furnish prior to or during formation of the sheet, a combination of cationic starch and starch phosphate, the combination having a zeta potential of from about +20 to xe2x88x9218 mV (millivolts).
In another embodiment of this invention, paper is made using the combination of cationic starch and starch phosphate having a select zeta potential as described herein, and wherein the starch phosphate is made by impregnating the starch with a phosphate reagent and then drying to substantially anhydrous conditions, preferably while in a fluidized state, prior to heat treating to effect phosphorylation. Preferably both the drying and heat treatment take place while in the fluidized state.
This invention involves a combination of modified cationic starches and starch phosphates in amounts to provide a selected zeta potential range for use in papermaking.
The modified starches which are used in this invention can be prepared by methods known and described in the art. Cationization of the starch can be produced by well known chemical reactions with reagents containing amino, imino, ammonium, sulfonium and phosphonium groups as disclosed, for example, in xe2x80x9cCationic Starchesxe2x80x9d by D. B. Solarek, Modified Starches: Properties and Uses, Chapter 8, pp. 113-129, 1986, and in U.S. Pat. No. 4,119,487 issued Oct. 10, 1978 to M. Tessler. Such cationic derivatives include those containing nitrogen groups comprising primary, secondary, tertiary and quaternary amines and sulfonium and phosphonium groups attached through either ether or ester linkages. The preferred derivatives are those containing the tertiary amino and quaternary ammonium ether groups.
The general method for preparing starches containing tertiary amine groups, which method involves reacting starch under alkaline conditions with a dialkylaminoalkyl halide is described in U.S. Pat. No. 2,813,093 issued on Nov. 12, 1957 to C. Caldwell, et al. Another method, therefore, is disclosed in U.S. Pat. No. 4,675,394 issued Jan. 23, 1987 to D. Solarek et al. The primary and secondary amine starches may be prepared by reacting the starch with aminoalkyl anhydrides, amino epoxides or halides, or the corresponding compounds containing aryl in addition to the alkyl groups.
Quaternary ammonium groups may be introduced into the starch by suitable treatment of the tertiary aminoalkyl ether or starch, as described in the previously noted U.S. Pat. No. 2,813,093. Alternatively, quaternary groups may be introduced directly into the starch by treatment with the reaction product of an epihalohydrin and a tertiary amine or tertiary amine salt, to provide, for example, (3-trimethylammonium chloride)-2-hydroxypropyl ether substituent groups as disclosed in the noted U.S. Pat. No. 4,119,487. The above noted patents, i.e., ""487, ""093 and ""394 are incorporated herein by reference.
The preparation of cationic sulfonium derivatives is described in U.S. Pat. No. 2,989,520 issued June, 1961 to M. Rutenberg et al. and essentially involves the reaction of starch in an aqueous alkaline medium with a beta-halogenoalkylsulfonium salt, vinylsulfonium salt or epoxyalkylsulfonium salt. The preparation of cationic phosphonium derivatives is disclosed in U.S. Pat. No. 3,077,469 issued Feb. 12, 1963 to A. Aszalos and involves reaction of starch in an aqueous alkaline medium with a beta-halogenoalkylphosphonium salt. The above noted patents, i.e. ""520 and ""469 are incorporated herein be reference.
Other suitable cationic starches may be provided using reagents and methods that are well known in the art as illustrated in the above references. Further description of useful cationic starches are disclosed in U.S. Pat. No. 2,876,217 issued Mar. 3, 1959 to E. Paschall, U.S. Pat. No. 2,970,140 issued Jan. 31, 1961 to C. Hullinger et al., U.S. Pat. No. 5,004,808 issued Apr. 2, 1991 to M. Yalpani et al., U.S. Pat. No. 5,093,159 issued Mar. 3, 1992 to J. Fernandez et al., and EP 406 837 published Jan. 1, 1991 (corresponding to U.S. application Ser. No. 516,024 filed Apr. 26, 1990), all of which are incorporated herein by reference. Particularly useful cationic derivatives are those containing amino or nitrogen groups having alkyl, aryl, aralkyl or cyclic substituents of up to 18 carbon atoms and especially alkyl of 1 to 6 carbon atoms.
The amount of cationic substituent on the starch can be varied and generally a degree of substitution (DS) of from about 0.003 to 0.2 and preferably from about 0.01 to 0.1 will be used. While larger amounts of cationic substituents or higher degrees of substitution (DS) could be used, they are more costly and difficult to make and therefore not economically attractive. The term xe2x80x9cdegree of substitutionxe2x80x9d (DS) as used herein means the average number of sites or substituent groups per anhydroglucose unit of the starch molecule.
The anionic starch used in this invention is a starch phosphate monoester. The starch phosphate can be prepared by phosphorylation using any known method including the reaction with various inorganic phosphate salts. The preparation of starch phosphate monoesters using such methods are described in xe2x80x9cPhosphorylated Starches and Miscellaneous Inorganic Estersxe2x80x9d by D. B. Solarek, Modified Starches: Properties and Uses, Chapter 7, pp. 97-112, 1986.
Phosphate groups are introduced into starch by thermal reaction with water-soluble ortho-, pyro-, meta-, or tripolyphosphates. Illustrative phosphate reagents are the alkali metal phosphates such as sodium and potassium ortho-phosphate, phosphoric acid, phosphorus oxychloride, sodium and potassium tripolyphosphate and sodium and potassium trimetaphosphate. The reagent may be either a mono-, di- or trialkyl metal phosphate or combination thereof.
Techniques for phosphorylating a starch base are further described in U.S. Pat. Nos. 2,824,870 issued Feb. 25, 1959 to H. Neukom and 2,961,440 issued Nov. 22, 1960 to R. Kerr. These patents disclose techniques of heat reacting starch impregnated with a phosphate salt of an alkali metal, within a prescribed pH range. Previously noted U.S. Pat. No. 3,562,103 directed to starches containing anionic phosphate groups, discloses a method of phosphorylating starch which comprises forming an aqueous starch slurry at room temperature and adding a suitable concentration of phosphate reagent. Preferably, the pH is adjusted to between 4 and 6, although it is stated that a range of 4 to 11.5 may be used. The starch is filtered without washing and adjusted to a moisture level of about 20% or below, preferably from about 5 to 20% by weight at a temperature of less than about 70xc2x0 C. The starch phosphate composition is then heated at a temperature of 100 to 160xc2x0 C. until the product has the desired level of anionic phosphate groups. The above noted ""870, ""440 and ""103 patents are all incorporated herein by reference.
In U.S. Pat. No. 4,166,173 issued Aug. 28, 1979 to O. B. Wurzburg et al., which disclosure is incorporated herein by reference, starch is phosphorylated by an improved pollution-free process which involves forming a concentrated reagent solution of alkali metal tripolyphosphate salt and impregnating therewith a starch cake containing no more than 45% by weight of moisture. Drying and thermally reacting the thus impregnated starch provides the phosphorylated starch. In preparing the concentrated reagent solution during addition of the tripolyphosphate salt to the water, one or more acids are added to control the pH at between 2.8 and 5.0.
For the purpose of this invention, any phosphate containing starch, native or modified may be used. For the modified product, the phosphorylation may be carried out by any of the known techniques with the thermal reaction of the phosphate impregnated starch being performed at a pH between 5.5 and 8.5, and preferably 6.0 to 8.5. The reaction of starch may be carried out, e.g., with sodium or potassium tripolyphosphate, sodium or potassium hexametaphosphate and sodium or potassium pyrophosphate salts yielding orthophosphate mono-ester groups, i.e., mono-starch phosphates. Other alkali metal salts may be used in place of sodium or potassium which are preferred as the phosphorylating reagent.
Thus, in carrying out phosphorylations employing an aqueous starch slurry, the pH of the starch slurry containing the phosphorylating reagent is adjusted to about 5.5 to 8.5. Use of pH levels below about 5.5 will result in a degraded starch while use of pH levels above about 8.5 may produce undesirable crosslinking. If phosphorylation is to be carried out by spraying the reagent, a starch slurry is ordinarily prepared and adjusted to be within the designated pH range and is then filtered. The reagent is sprayed onto the pH adjusted starch cake. The practitioner will recognize that it is also possible to prepare the filter cake at a slightly alkaline pH and impregnate it with an acidic solution of phosphate reagent such that the final pH of the starch-phosphate reagent mixture is within the defined pH range. The specific reagent used may require adjustments of pH levels. For example, sodium tripolyphosphate (STP) has limited solubility in water (0.2 g/cc at 25xc2x0 C). In order to achieve higher solids solutions, the pH is maintained at 4.0 to 6.0 by addition of acid such as HCI or H3PO4 during dissolution of the salt. In contrast, sodium hexametaphosphate (NaPO3)6 shows very high solubility and concentrated solutions (20 to 36%, by weight) can be prepared with no pH adjustment. Suitable starch phosphate for this invention will include about 0.03 to 1.0% bound phosphorus, preferably about 0.1 to 0.5% by weight, based on the weight of dry starch. By the term xe2x80x9cbound phosphorusxe2x80x9d we mean phosphorus which is attached by an ester linkage to a hydroxyl group of the anhydroglucose backbone of the derivatized starch. Bound phosphorous may also be defined as the phosphorus that cannot be removed from the product by conventional washing or separation techniques.
Most commonly, the amount of phosphorylating reagent employed will range from about 0.5 to 12% by weight, based on weight of dry starch. For example, treatment of waxy maize with 3.5 to 4.0% of sodium tripolyphosphate will give a starch containing 0.14 to 0.22% of bound phosphorus. The starch cake containing the phosphorylating reagent is dried to a moisture less than about 9.0% and preferably from about 2.0 to 7.0% prior to the required thermal treatment at higher temperatures. Ordinarily, the dry mixture of starch and phosphorylating reagent is heated to temperatures of from about 110xc2x0 to 140xc2x0 C. and preferably will range from about 130xc2x0 to 135xc2x0 C. during the phosphorylation reaction. The heating period may range from 0.1 to 4 hours or more depending on the selected reagent, pH, temperature, etc. The phosphorylating operation is performed under conditions which avoid severe molecular degradation, as would be shown by a significant decrease in viscosity of the starch.
While, as described above, any phosphate containing starch may be used in this invention, a particularly useful starch phosphate is one that is made by impregnating the starch with phosphate and then dried to substantially anhydrous conditions before heat treating to effect phosphorylation. By anhydrous or substantially anhydrous conditions is meant less than about 1% by weight moisture content, based on the dry weight of starch. It is preferred that both the drying and phosphorylation steps take place in the fluidized state. While other drying and phosphorylating systems may be used which are not in the fluidized state, the fuidized state is preferred because it provides excellent heat and mass transfer which results in good and desired drying and reaction characteristics.
Impregnation by the phosphate reagent may be accomplished by adding the reagent, at a level of less than about 15% and preferably less than about 10% by weight, based on the dry weight of starch, either in the dry state or to the wet starch, or by dissolving the reagent in water to form an aqueous solution which is then mixed with the starch. These impregnation techniques are described in U.S. Pat. No. 4,166,173 noted above, and U.S. Pat. No. 4,216,310 issued on Aug. 5, 1980 to O. Wurzburg et al., both of which are incorporated herein by reference.
The impregnated starch is first subjected to the fluidized state and dried to anhydrous conditions of less than about 1% by weight moisture content based on the weight of starch, at less than about 140xc2x0 C., more particularly between about 60 to 140xc2x0 C., preferably between about 100 to 125xc2x0 C. The dried product, while still in the fluidized state, is heated to between about 100 to 185xc2x0 C., preferably between about 120 to 140xc2x0 C. for about 30 to 300 minutes. At temperatures higher than about 150xc2x0 C., the processing time is preferably less than about 45 minutes.
The method and conditions for impregnating and phosphorylating starch for the operation where the impregnated starch is dried to anhydrous conditions and then phosphorylated while in the fluidized state, can be the same as the known procedures desribed previously.
The fluidized state is achieved by vigorous mixing of the solid starch particles in vacuum or in a gas whereby a uniform distribution of starch throughout the vacuum or gas may be attained. Vigorous mixing may be accomplished by using air or gas, at or above atmospheric pressures in a fluidized bed reactor or by sufficient mechanical agitation. Where pressurized gas is used to effect the fluidized state, the velocity of the gas must achieve a minimum rate such that the particles are free to move and exhibit a xe2x80x9cfluidized statexe2x80x9d. The fluidized state results in very efficient heat transfer and allows the starch to rapidily dry to a virtually anhydrous state at low temperature.
The phosphate esters prepared by predrying to anhydrous conditions while in the fluidized state, as described above, are characterized by improved purity as the high reaction efficiency provides a product with a high level of substitution while providing a low level of residual inorganic phosphate salts in the final starch phosphate monoester products. Further, the process minimizes undesirable side reactions such as starch hydrolysis and crosslinking. Such products are also characterized by improved viscosity, color and uniformity. The new process also preserves the granular integrity of the starch advantageously allowing the optional washing of the final product.
The starch phosphate prepared, as described above, using anhydrous conditions and the fluidized state involves a process having improved reaction efficiency. Reaction efficiency is defined to be the amount of bound phosphorous divided by the total amount of phosphorous used in the process, multiplied by 100. Phosphorus content can be measured by any suitable conventional analytical technique such as inductively couple plasma (ICP) or gravimeetric analysis. Reaction efficiencies of this process may be as high as about 70 to 85% or more, and are much improved over the reaction efficiency of traditional methods of phosphorylating starches.
The process of preparing starch phosphates by impregnating starch with a phosphate reagent and then drying to anhydrous conditions while in a fluidized state prior to heat treating is described in further detail in copending U.S. patent application #1878, entitled xe2x80x9cImproved Starch Phosphates Ester Composition, Process and Method of Use in Foodxe2x80x9d with Wolfgang Bindzus et al. as inventors, filed on the same date as this application. The process and details of this process as described in this #1878 U.S. application are hereby incorporated herein by reference.
The starch which may be used as the base material in preparing the modified cationic starch and anionic starch materials of this invention may be derived from any plant source including corn, potato, wheat, rice, waxy rice, tapioca, waxy maize, sago, sorghum, high amylose starch such as amylose corn having 40% or more of amylose content, etc. Starch flours may also be used. Also included are the conversion products derived from any of the former bases including, for example, dextrins prepared by the hydrolytic action of acid and/or heat; oxidized starches prepared by treatment with oxidants such as sodium hypochlorite; fluidity or thin-boiling starches prepared by enzyme conversion or mild acid hydrolysis; derivatized or modified starches; and crosslinked starches. Preferred starches are waxy maize, corn, tapioca, potato starch and combinations thereof. The starch base may be a granular starch or a gelatinized starch, i.e., non-granular starch. It is further noted that the starch base material may be the same or different for each of the cationic starch or anionic starch components.
While the amount of modification, i.e., the cationic substitution and anionic substitution on the respective starch components can vary as noted above, the essential feature of this invention is that the respective combinations of the two components are provided in proportions such that the net zeta potential of the starch combination will be in the range of from about +20 to xe2x88x9218 mV and preferably from about +15 to xe2x88x925 mV. Keeping the zeta potential within this range is important because when using the starch combination in a papermaking process, significant improvement is seen in filler retention as well as good drainage and strength properties.
When the papermaking process employing the cationic starch and anionic starch phosphate combination in accordance with this invention involves an alkaline papermaking system, i.e., where the pH of the system is typically greater than about 6.5, the starch combination will more particularly have a zeta potential of from about +18 to xe2x88x9218 mV and preferably from about +15 to xe2x88x9210 mV. When the papermaking process involves an acid papermaking system, i.e., pH of less than about 6.5, the cationic starch/anionic starch phosphate combination will have a zeta potential of from about +20 to +1 mV and preferably from about +17 to +5 mV.
The proportion or ratio of cationic starch and starch phosphate components in the starch combination of this invention can vary to a wide degree provided that the zeta potential range, as described herein, is satisfied. More particularly, the cationic starch and starch phosphate polymer components are generally provided in amounts of from about 4:1 to 1:4 parts by weight of cationic starch to starch phosphate. The term xe2x80x9czeta potentialxe2x80x9d as used herein refers to electrokinetic potential, the potential across the interface of solids and liquids and more particularly the potential across the diffuse layer of ions surrounding a charge colloidal particle. Zeta potential relates to surface charge and electrophoretic mobility and is a well known property measurement. A detailed discussion of zeta potential may be found in xe2x80x9cZeta Potential in Colloid Science, Principles and Applicationsxe2x80x9d by Robert J. Hunter, Academic Press, 1988. Various methods are known for determining the zeta potential of different materials, with electrophoresis being the most common. Electrokinetic phenomena are observed when two phases move relative to each other under the influence of an electric field. Electrophoresis describes the motion of charged particles or droplets in an applied electric field. A thin layer of liquid adheres to the particle or droplet surface so that the shearing plane is located anywhere in the liquid. The potential at the shearing plane is called the electrokinetic or zeta potential. Zeta potential can readily be measured by the technique of microelectrophoresis. This involves measuring the velocity of individual particles in suspension, viewed in a microscope fitted with a reticule, the transit times across the reticule being recorded. Several microelectrophoresis instruments are available for measuring zeta potential. In this application, a Zetasizer 2000 instrument provided by Malvern Instruments Limited was used to measure zeta potential.
The combination of cationic starch and starch phosphate may be effectively used for addition to pulp prepared from any type of cellulosic fibers, synthetic fibers, or combinations thereof. Among the cellulosic materials which may be used are bleached and unbleached sulfate (kraft), bleached and unbleached sulfite, bleached and unbleached soda, neutral sulfite, semi-chemical, chemiground wood, ground wood or any combination of these fibers. Fibers of the viscose rayon or regenerated cellulose type may also be used if desired.
Any desired inert mineral fillers may be added to the pulp which is to be modified with the improved starch derivatives of this invention. Such materials include clay, titanium dioxide, talc, calcium carbonate, calcium sulfate and diatomaceous earths.
Other additives commonly introduced into paper may be added to the pulp or furnish, for example, dyes, pigments, sizing additives, alum, anionic retention aids, etc.
The amount of cationic starch and starch phosphate polymer combination (i.e. the total amount of both the cationic starch and starch phosphate components) that may be added to the wet end or paper pulp will be an effective additive amount, especially effective to improve filler retention. More particularly, from about 0.05 to 10% of the starch combination and preferably from about 0.1 to 2% by weight, based on the dry weight of the stock or furnish, may be used. The starch combinations are made as a component, i.e. the cationic starch and starch phosphate are combined together as one component to form the starch polymer combination. These starch materials or combination may be added to the papermaking system in the cooked or uncooked condition. If they are cooked, i.e. dispersed or solubilized, this may be accomplished by standard or known techniques such as batch cooking, jet cooking or steam injection cooking. The starch components can either be cooked together as a blend or cooked separately, then blended and combined together and added as one component to the papermaking system. When the starch materials are cooked, the desired paper characteristics may be achieved by using lesser amounts of starch.
In addition to the selected starch derivatives and other components that may be included in the alkaline papermaking system as described above, colloidal inorganic minerals may be added to the system to form an alkaline microparticle system. Such microparticle systems may include colloidal silica or bentonite, and alum and may be incorporated into the system in amounts of at least 0.001% and more particularly from about 0.01 to 1% by weight based on the weight of dry pulp. Further description of such microparticle inorganic materials may be found In U.S. Pat. No. 4,388,150 issued Jun. 14, 1983; U.S. Pat. No. 4,643,801 issued Feb. 17, 1987; U.S. Pat. No. 4,753,710 issued Jun. 28, 1988 and U.S. Pat. No. 4,913,775 issued Apr. 3, 1990; all of which are incorporated herein by reference.